1. Field of the Invention
This invention relates to such image forming apparatus as electrophotographic apparatus, printers, and the like to obtain color developer images using a revolver type-developing unit.
2. Description of the Related Art
The image forming apparatus produces a full-color image using 4 color toners of yellow (Y), magenta (M), cyan (C) and black (BK). In this image forming apparatus, a photoconductor is rotated by plural turns and latent images in respective colors are formed successively on the photoconductor for every turn. After forming the latent images on the photoconductor, these latent images are made visible by supplying toners in different colors to the photoconductor by plural developing units and the formed toner images are superposed on each other on the photoconductor or an intermediate transfer body. Thus, the image forming apparatus is made small in size.
As developing units to supply toners in different colors successively to latent images formed on a photoconductor for every turn of the photoconductor, a 4-color revolver type developing unit for developing latent images on the photoconductor by arranging developing units opposing to the photoconductor for supplying developers in specified colors successively by revolving a single revolving holder equipped with four developing units having four color toners of yellow (Y), magenta (M), cyan (C) and black (BK) is disclosed in Japanese Patent Publication No. 2001-24952. However, this 4-color revolver type developing unit is small in size but sizes of developing units that are incorporated in a revolving-type holder are limited. Therefore, a black developing unit to supply black (BK) toner only consumes much black toner fast and replenishment of black toner or exchange of a black developing unit is frequently demanded.
Therefore, an apparatus equipped with three developing units only to supply three colors of yellow (Y), magenta (M) and cyan (C), and a large capacity black developing unit for supplying much consuming black (BK) toner arranged separately from a revolver type developing unit for extending the life of the black developing unit is disclosed in Japanese Patent Publication No. 6-19271.
However, in the case of this 3-color revolver type developing unit equipped with 3-color developing units of yellow (Y), magenta (M) and cyan (C) only and a black developing unit provided separately, 3-color developing units 1Y, 1M, and 1C for supplying yellow (Y), magenta (M) and cyan (C) are arranged at an angle 120xc2x0 to a revolving shaft 1a of the 3-color revolver type developing unit as shown in FIG. 1. That is, in this 3-color revolver type developing unit 1, to switch the yellow (Y) developing unit 1Y to the next magenta (M) developing unit 1M after completing the yellow toner image development, it is necessary to revolve the 3-color revolver type developing unit 1 by 120xc2x0.
In the case of a 4-color revolver type developing unit 2 shown in FIG. 2, four color developing units of yellow 2Y, magenta 2M, cyan 2C and black 2BK are arranged at an angle 90xc2x0 to the revolving shaft 2a. Therefore, the developing units can be switched by revolving the 4-color revolver type developing unit 2 only by 90xc2x0 but the 3-color revolver type developing unit 1 requires 4/3 times of revolving amount.
For this reason, when the printing velocity of the 3-color revolver type developing unit 1 is made at least the same as that of the 4-color revolver type developing unit 2, it is necessary to revolve the 3-color revolver type developing unit at a higher speed of 4/3 times than the revolving speed of the 4-color revolver type developing unit 2.
Moreover, the further high-speed process is demanded in recent years for the increase in the number of image forming sheets per unit time resulting from the pursuit of high productivity of an image forming apparatus. This high productivity of the number of image forming sheets is achieved by increasing the processing speed and decreasing the supply interval of recording paper during the image forming process. In order to reduce the supply interval of recording paper, a switching time of developing units is more and more reduced, and the more increase of revolving velocity in switching the developing units is demanded for the revolver type developing unit.
For such the increase of switching velocity of the developing units, a driving motor and a power source for revolving the revolver type developing unit become large and the development of small-sized and energy saving image forming apparatus is impeded and further, vibration is generated when the revolver type developing unit is stopped after the high-speed revolving, and this vibration may possibly adversely affect images.
Accordingly, in an image forming apparatus to obtain color developed images using a revolver type developing unit, it is expected to develop a small-sized driving motor and a power source for a revolver type developing unit to realize the high-speed development without impeding the maintenance or the long life of a black developing unit and to obtain images of good quality without being adversely affected by vibration when stopping the revolver type developing unit.
An object of this invention is to provide an image forming apparatus with a driving motor and a power source small in size without impeding the high-speed image forming process by a revolver type developing unit with a black developing unit arranged separately, capable of revolving in a short time and obtaining good images without generating vibration.
According to the embodiment of this invention, an image forming apparatus is provided. This image forming apparatus comprises an image carrier; a latent image forming unit to form latent images on the image carrier; a black developing unit arranged near the image carrier to supply a black developer to the image carrier; and a revolving-type developing unit. This revolving-type developing unit is arranged at a position near the image carrier and differing from the black developing unit, comprising developing units of plural color developing units which have different color developers arranged at equal space of a first distance along the circumference of a revolving body in contact with the image carrier. On the other hand, the developing unit at the last part in the revolving direction of the revolving body to the developing unit at the foremost part in the revolving direction are separated at a second distance that is longer than the first distance, and anyone of the plural color developing units is arranged opposing to the image carrier and supplies a color developer, and it is characterized in that for the area from the developing unit at the last part of the revolving-type developing unit to the foremost developing unit separated by the second distance opposing to the image carrier, a black developer is supplied to the image carrier by the black developing unit.
Further, according to the embodiment of this invention, an image forming apparatus is provided. This image forming apparatus comprises an image carrier; a latent image forming unit to form latent images on the image carrier; a black developing unit arranged near the image carrier to supply a black developer to the image carrier; and a revolver type developing unit arranged at a position differing from the black developing unit. This revolver type developing unit is composed of plural color developing units having different color developers, and developing rollers of these plural color developing units are arranged along the circumference of a revolving holder in contact with the image carrier at an equal space of the first distance and on the other hand, those developing rollers from the roller at the last part in the the revolving direction of the revolving body to the roller at the foremost part in the revolving direction are separated at a second distance that is longer than the first distance, and one of the plural color developing units is arranged opposing to the image carrier sequentially and supplies a color developer to the image carrier, and it is characterized in that the area ranging from the developing roller at the last part of the revolving-type developing unit to the foremost developing roller separated by the second distance opposing to the image carrier, a black developer is supplied to the image carrier by the black developing unit.